Flux gate gradiometer using a pulse generator to drive the energizing windings as well as actuate the detector

ABSTRACT

A gradiometer is provided utilizing two flux gate magnetometers to detect magnetic field strength. The signal is sensed only during the magnetic core saturation drive pulse portion of the cycle, the core driver circuitry acting to control sense windings and drive windings simultaneously. There is provided unidirectional drive of the core, a low duty cycle drive, and interrogation of the sense winding only during the drive pulse. The unidirectional drive of the core and low duty cycle conserve power.

United States Patent Van Sloun et al.

represented by the Secretary of the Air Force [22] Filed: Jan. 27, 1972[21] Appl. No.: 221,474

[52] US. Cl. 324/43 G [51] Int. Cl G0lr 33/04 [58] Field of Search324/43 R, 43 G, 47

[5 6] References Cited UNITED STATES PATENTS 3,286,169 11/1966Slonczewski 324/47 3,541,432 11/1970 Scarbrough... 3,649,908 3/1972Brown 324/43 R COMMoN MonE F E ED Bnc K PULSE FLUX GATE GRADIOMETERUSING A PULSE GENERATOR TO DRIVE THE ENERGIZING WINDINGS AS WELL ASACTUATE THE DETECTOR Inventors: Peter H. Van Sloun, Hopkins,

Minn.', Burton A. Pearlstein, Sharon, Mass.

Assignee: The United States of America as OTHER PUBLICATIONS Morris etal., Design of Second Harmonic Flux Gate Magnetic Field Gradiometer,Rev. of Scien. lnst., Vol. 32, No. 4, April 1961, pp. 444-448 PrimaryExaminerRobert J. Corcoran Att0rneyHarry A. Herbert, Jr. et a1.

ABSTRACT A gradiometer is provided utilizing two flux gate magnetometersto detect magnetic field strength. The signal is sensed only during themagnetic core saturation drive pulse portion of the cycle, the coredriver circuitry acting to control sense windings and drive windingssimultaneously. There is provided unidirectional drive of the core, alow duty cycle drive, and interrogation of the sense winding only duringthe drive pu1se. The unidirectional drive of the core and low duty cycleconserve power.

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snmsors IFIE ,6 (msv) FLUX GATE GRADIOMETER USING A PULSE GENERATOR TODRIVE THE ENERGIZING WINDINGS AS WELL AS ACTUATE THE DETECTOR BACKGROUNDOF THE INVENTION This invention relates to apparatus for measuring orsensing magnetic fields and more particularly to a magnetic gradiometerutilizing two flux magnetometers to detect magnetic field strength.

The gradiometer of the present invention employs flux gatemagnetometers. The flux gate magnetometers are different from the wellknown second harmonic flux gate magnetometers in the techniques of usingpulsed saturation of the magnetic core in a single direction and sensingthe signal only during the saturation drive pulse.

A conventional flux-gate magnetometer consists of a saturable, highpermeability core with a drive coil and a pickup coil wound around it.The drive coil is used to drive the core into saturation alternately inthe two opposite directions along the core axis. The flux linesthreading through the pickup coil, then, are the sum of those created bythe drive coil and those due to the ambient magnetic field. Since thedrive coil saturates the core alternately in the two directions colinearwith the core axis, the contributions of the ambient flux to the totalflux threading the pickup coil is negligible except at the times atwhich the sense of saturation is reversing; that is, except when theambient flux is gated on" exactly at the cross-over time or the times onthe flux versus time curve when the flux B crosses the time axis. Theeffect of the ambient flux is to shift the crossover instants. When theambient field is zero, the crossover instants are evenly spaced and thissymmetry is upset by the ambient flux when the ambient field is otherthan zero. In that case, the cross-over points on the negative halfcycle on the flux versus time curves are wider spaced than those on thepositive half cycle. The curve of flux B versus time with H containsonly even harmonics, whereas the curve with H present contains oddharmonics as well as even harmonics. One disadvantage of theconventional flux-gate magnetometer is that it is limited in accuracy,as opposed to sensitivity, by the remnant magnetization of the core,which introduces an uncertainty in the zero level. Another disadvantageis the limitation placed on the sensitivity by certain sources of noisesuch as the resistance of the output coil, the reflected resistance fromthe drive coil, and the Barkhausen noise in the core material.

The present gradiometer utilizes two flux gate magnetometers to detectthe magnetic field strength. The unique characteristics employed inoperating these magnetometers include: unidirectional drive of the core,use of a low 1:41 duty cycle drive, and interrogation of the sensewinding only during the drive pulse. The unidirectional drive of thecore and low duty cycle conserve power.

SUMMARY OF THE INVENTION The gradiometer of the present inventionutilizes two flux gate magnetometers to detect magnetic field strength.The unique features employed in operating these magnetometers include:unidirectional drive of the core, use of a low duty cycle drive, andinterrogation of the sense winding only during the drive pulse. Thesignal representative of the magnetic field strength is derived from thesense winding by virtue of the flux lines C which were inside of thewinding relaxing to the outside when the core is saturated. Signalvariations are caused by changes in the number of flux linesconcentrated through the core material.

A pulse generator and core driver is used to drive the windings on thecore and also to switch sense windings onto the circuitry during thedrive pulse. The sense windings drive directly into a differentialamplifier. Common mode feedback is used to buck out the ambient field inwhich the sensor rests. A balance feedback is used to null out anygradient that may be seen as the sensor comes to rest. A signalamplifier increases the signal level of the output from the differentialamplifier.

DESCRIPTION OF THE DRAWINGS FIGS. la-le illustrate the flux gatefunction;

FIG. 1A is a graphicrepresentation of the core;

FIG. 1B shows the core material as it is in the unexcited state;

FIG. 1C shows the sensor with the drive windings excited;

FIG. 2 shows a block diagram of the gradiometer;

FIG. 3 is a schematic diagram of pulse generator and core driver 10 ofFIG. 2;

FIG. 4 illustrates a schematic diagram of the differential amplifier ofFIG. 2;

FIG. 5 shows in schematic diagram form the common mode feedbackcomponent of FIG. 2;

FIG. 6 is a schematic diagram of the balance feedback component of FIG.1; and

FIG. 7 is the amplifier schematic.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The gradiometerutilizes two flux gate magnetometers to detect the magnetic fieldstrength. The unique characteristics employed in operating thesemagnetometers include: unidirectional drive of the core, use of a low1:41 duty cycle drive, and interrogation of the sense winding onlyduring the drive pulse. The unidirectional drive of the core and lowduty cycle conserve power.

A graphic representation of the core and its function are given in FIGS.1A, 1B, and 1C. Referring to FIG. 1A, core material 2 is mo-permalloy4-79. This is a high permeable material. Drive windings 3 are wrappedaround core material 2 and saturate it when excited. This excitationeffectively reduces the permeability of the core as viewed from theoutside from very high to approximately one. Sense winding 5 is woundaround bobbin 4 as shown. When assembled, core 2 is inside sense windingbobbin 4.

View of FIG. 1B shows the core material as it is in the unexcited state.That is, the permeability as viewed from the outside is very high, thusthe flux lines are concentrated through it as shown.

View of FIG. 1C shows the sensor with the drive windings excited, thushaving the core material fully saturated. In this condition thesaturated core permeability looks as if it is reduced to nearly one asobserved from the outside. Thus, the flux lines are not concentratedthrough it. Note that the drive windings drive one-half of the core upand the other half down thus cancelling the resultant coupling to thesense winding. FIG. 1C also illustrates one assembly, however there aretwo assemblies provided with each assembly having an associated feedbackwinding.

The signal representative of the magnetic field strength is derived fromthe sense winding by virtue of the flux lines which were inside of thewinding relaxing to the outside when the core is saturated. Signalvariations are caused by changes in the number of flux linesconcentrated through the core material.

For ease in presentation of the operation of the gradiometer, a blockdiagram of it is given in FIG. 2. Pulse generator and core driver isused to drive the windings on the core and also to switch the sensewindings onto the circuitry during the drive pulse. The sense windingsdrive directly into differential amplifier 12. Common mode feedback 13is used to buck out the ambient field in which the sensor rests.

Balance feedback 14 is used to null out any gradient that may be seen asthe sensor comes to rest. Signal amplifier 15 increases the signal levelfor the next section. Each block is explained with the use of aschematic diagram hereafter.

The pulse generator and core driver schematic diagram is shown on FIG.3. This section drives the core into saturation with a pulse width of 12microseconds every 500 microseconds. The 12 microseconds pulses are alsofed to common mode feedback 13 and to differential amplifier 12 and usedthere in switching. Resistor and capacitor 21 provide decoupling fromthe power supply which is required due to the pulse type loading of thecores. A basic 2 kilohertz oscillator is formed by a programmableunijunction transistor 22 and its associated circuitry. Resistor 23 andcapacitor 24 form the basic frequency. Transistor 25 is used to triggerthe monostable multivibrator which generates the 12 microsecond widthpulse. The monostable multivibrator is formed by transistors 26 and 27along with their associated circuitry. Transistors 28 and 29 amplify thesignal to drive windings T151 and T281 on the cores of two flux gatemagnetometers. Diode 18 and capacitor 20 are in parallel across windingTlBl and T281. Terminal 19 provides output pulse A.

The differential amplifier schematic diagram is given in FIG. 4.Operational amplifier 30 is used in this circuit. Its inputs are fromtwo sensor windings T182 and T2B2, one from each sensor core. Resistors31 and 32 are connected directly across the sense windings to optimizetheir output. Resistor 55 is used in balance feedback of FIG. 6 to biasthe sense winding from core No. 2 up or down to balance out any gradientthat may be present. In operation both sense windings put out a voltagesignal to the operational amplifier. The difference between the twosignals is amplified. Transistor 33 is a field effect transistor used asa switch. Its switching signal comes from terminal 19 of pulse generatorand core driver 10 by way of terminal 16. The switch 33 is turned ononly during the 12 microsecond pulse. The on resistance of transistor 33and capacitor 34 forms an integrator to remove the amplitude modulationof the signal. This signal is transmitted to the signal amplifier. Thesense winding outputs are fed to the common mode feedback section inaddition to feeding the differential amplifier. The outputs of sensewindings T182 and T282 are available at terminals 35 and 36,respectively.

A schematic diagram of the common mode feedback section is given in FIG.5. Resistors and 41 receive the signal from the sense windings by way ofterminals 37 and 38 from terminals 35 and 36, respectively, of FIG. 4,and take the average between the two. This signal is switched on bytransistor 42 only during the 12 microseconds pulse upon the receipt atterminal 45 of a pulse from terminal 19 of FIG. 3. The objective of thisfeedback section is to reduce its input to near zero by means of thefeedback winding via magnetic feedback. This feedback bucks out theambient field. The operational amplifier 43 is used in a sample and holdconfiguration. This is achieved by capacitor 44 and resistor 46 in itsfeedback loop. The amplifier can drive the windings T183 and T283 sothat they produce a bucking field in either direction, thus effectivelycancelling the earth's ambient field. The earths ambient field iscancelled for two reasons: first, to reduce the signal level output atthe sense windings so that the common mode rejection feature of theoperational amplifier is not exceeded; and secondly, to reduce the noiselevel of the signal.

The balance feedback component of FIG. 2 is used to keep the signalwithin a good dynamic range and establish a zero level gradient at theoutput. It also cancels a gradient in the field that the gradiometer maybe required to operate in. A schematic diagram is given in FIG. 6.Resistor 50 and capacitor 51 form a very long time constant whichprevents signals in the desired frequency range from effecting thebalance feedback section. A field effect transistor 52 is used forimpedance matching. An operational amplifier 53 is used to drive thebalance feedback in the differential amplifier section. This feedbackestablishes the voltage level from which the second core sense windingwill operate from, thus establishing a zero output in the quiescentcondition. It is noted that terminal 49 receives a signal from signalamplifier 15 of FIG. 2 and terminal 54 provides an output to terminal 55of the differential amplifier of FIG. 4.

The signal amplifier of FIG. 2 is used to increase the signal voltageand to limit its frequency response. It has a frequency cutoff atapproximately 5 hertz. The amplifier schematic is shown in FIG. 7. Anoperational amplifier is used in the standard low pass configuration.The input signal from output terminal 61 of the differential amplifierof FIG. 4 is received at input terminal 61 and the output signal fromterminal 62 is fed to terminal 49 of the balance feedback component ofFIG. 6.

It is noted that two flux magnometers are utilized to detect themagnetic field strength. There are a pair of assemblies utilized, one ofwhich is illustrated in FIG. 1C. The schematic representation of thewindings are illustrated in FIG. 3 which shows drive windings T181 andT28]. FIG. 4 shows the connections for sense windings T182 and T282.Also provided are a pair of feedback windings associated with the fluxmagnetometers for bucking out the ambient field. They are shown as T183and T283 of FIG. 5. The foregoing windings have been illustrated aspreviously described for purposes of clarity although they actually areshown in the assembly of FIG. 1C.

What is claimed is:

l. A gradiometer to detect magnetic fields comprising first and secondflux gate magnetometers serving as a sensor for magnetic fields, each ofsaid flux gate magnetometers including a magnetic core, a drive winding,and a sense winding, said drive being connected in a series arrangement,pulse generating means providing pulses having a preselected timeduration at a predetermined pulse repetition rate, said pulses drivingsaid series arrangement, a differential amplifier having first andsecond inputs, said first input receiving the output signal of one sensewinding, and the second input the output signal from the other sensewinding, a first switch, normally off, in the output path of saiddifferential amplifier, said first switch connected to said pulsegenerator and upon receipt of pulses from the generator means beingturned on for the duration of each received pulse to permit outputsignals from said differential amplifier to pass therethrough, meansconnected to said sense windings to generate a bucking field in eitherdirection for said two flux gate magnetometers to cancel out the earthsambient field, means connected to said first switch to amplify thedifference signal from said differential amplifier to provide an outputsignal representative of the detected magnetic field, and meansconnected to said means to amplify to establish the voltage level whichsaid second sense winding operates from thus permitting theestablishment of a zero output in the quiescent condition, said meansestablishing said voltage level including a resistor and capacitornetwork receiving said signal representative of said detected magneticfields, said network having a long time constant, an amplifier,transistor impedance matching means interconnecting said network andsaid amplifier, the output signal from said amplifier being received byone of the inputs to said differential amplifier.

2. A gradiometer as described in claim 1 wherein the means to generate abucking field is comprised of means to average the signals from saidsense windings, switch means connected to said pulse generator and saidmeans to average and actuated by said pulses from said pulse generatingmeans to switch on the average signal during said preselected timeduration, an amplifier receiving the average signal, a pair of feedbackwindings associated with said two flux gage megnetometers, said feedbackwindings receiving the amplified average signal.

1. A gradiometer to detect magnetic fields comprising first and secondflux gate magnetometers serving as a sensor for magnetic fields, each ofsaid flux gate magnetometers including a magnetic core, a drive winding,and a sense winding, said drive being connected in a series arrangement,pulse generating means providing pulses having a preselected timeduration at a predetermined pulse repetition rate, said pulses drivingsaid series arrangement, a differential amplifier having first andsecond inputs, said first input receiving the output signal of one sensewinding, and the second input the output signal from the other sensewinding, a first switch, normally off, in the output path of saiddifferential amplifier, said first switch connected to said pulsegenerator and upon receipt of pulses from the generator means beingturned on for the duration of each received pulse to permit outputsignals from said differential amplifier to pass therethrough, meansconnected to said sense windings to generate a bucking field in eitherdirection for said two flux gate magnetometers to cancel out theearth''s ambient field, means connected to said first switch to amplifythe difference signal from said differential amplifier to provide anoutput signal representative of the detected magnetic field, and meansconnected to said means to amplify to establish the voltage level whichsaid second sense winding operates from thus permitting theestablishment of a zero output in the quiescent condition, said meansEstablishing said voltage level including a resistor and capacitornetwork receiving said signal representative of said detected magneticfields, said network having a long time constant, an amplifier,transistor impedance matching means interconnecting said network andsaid amplifier, the output signal from said amplifier being received byone of the inputs to said differential amplifier.
 2. A gradiometer asdescribed in claim 1 wherein the means to generate a bucking field iscomprised of means to average the signals from said sense windings,switch means connected to said pulse generator and said means to averageand actuated by said pulses from said pulse generating means to switchon the average signal during said preselected time duration, anamplifier receiving the average signal, a pair of feedback windingsassociated with said two flux gage megnetometers, said feedback windingsreceiving the amplified average signal.